π-Bond Screening in Benzonorbornadienes: The Role of 7-Substituents in Governing the Facial Selectivity for the Diels-Alder Reaction of Benzonorbornadienes with 3,6-Di(2-pyridyl)-s-Tetrazine

Benzonorbornadiene 21, 7-spirocyclopropylbenzonorbornadiene 23, 7,7-dimethylbenzonorbornadiene 25, and 7-spirocyclopentylbenzonorbornadiene 27 have been reacted with 3,6-di(2-pyridyl)-s-tetrazine (rate: 21>23>25=27) to form symmetrical 4,5-dihydropyridazines which are stable towards fragmentation but rearrange with varying facility to their 1,4 isomers. The facial selectivity of attack on the π-bond changes from exo-attack for 21 and 23 to endo-attack for 25 and 27. The 7-spirocyclopropyl benzonorbornadiene 23 typically forms a mixture of dihydropyridazines with exo-stereochemistry, which undergo further stereochemical isomerisation to an exo-fused product upon acetylation (acetyl chloride in hot pyridine). Oxidation with DDQ of the dihydropyridazines individually or as mixtures gives the corresponding fused 3,6-di(2-pyridyl)pyridazines.